The present invention relates to amplifier circuits. More specifically, the present invention relates to a feed forward amplifier.
Feed forward amplifiers (xe2x80x9cFFAxe2x80x9d) are well known. Within the art, it is also known to improve the linear behavior of FFAs using a pilot tone. Put in simple terms, a pilot tone is generated by a pilot tone generator and injected prior to the main amplifier. The injected pilot tone is then detected by a pilot tone receiver at the output of the circuit. The detected pilot tone is used to adjust the gain and phase of the output of the correctional amplifier, such that the pilot tone and error or distortion introduced by the main amplifier are substantially eliminated. The overall result is a substantially linear feed forward amplifier. More detailed explanations of pilot tone usage in FFAs can be found in a variety of sources, such as in U.S. Pat. No. 5,874,850 to Van Horn.
While the use of a pilot tone can improve the overall linearity of FFAs, current pilot tone generators and receivers tend to add complexity and overall cost to the FFA circuit. For example, current FFA circuits that utilize a pilot tone typically require multiple oscillatorsxe2x80x94i.e., a first oscillator for the pilot tone generator, and a second oscillator for the pilot tone receiver. Furthermore, these two oscillators must be programmed with complementary frequencies: namely, a first frequency for the first oscillator, and a second frequency for the second oscillator, whereby the second frequency differs from the first frequency by the intermediate frequency (xe2x80x9cIFxe2x80x9d) of the pilot tone receiver. Another problem with using two oscillators is that phase noise, (jitter in the time domain) can exist between the oscillators, thus requiring additional care, cost and/or complexity when designing both the generator and receiver to compensate for any phase noise.
Yet another problem with the prior art is that certain elements in the pilot tone receiver, such as the filter, can be difficult to appropriately size so that they properly complement the correctional amplifier and delay element used in the error signal path of the FFA circuit, thus increasing the overall complexity and cost of designing the FFA.
It is an object of the present invention to provide a novel feed forward amplifier that obviates or mitigates at least one of the above-identified disadvantages of the prior art.
In an aspect of the invention, there is provided a pilot tone generator-receiver for a feed-forward amplifier (FFA). The feed forward amplifier includes a main amplifier and an error correction pathway. The generator-receiver comprises an oscillator for producing a signal and a vector modulator connected to the oscillator. The vector modulator is operable to either add or subtract a modulating frequency to or from, respectively, the signal in order to generate a pilot tone that is suitable for injection prior to the main amplifier. The generator-receiver also includes a receiver circuit connected to the oscillator, which is for detecting a pilot tone from an output of the feed-forward amplifier. The receiver circuit is also operable to use the detected pilot tone to modify the controls of the error correction pathway, such that the error pathway is then operable introduce a signal at the output of the feed-forward amplifier that substantially eliminates the pilot tone and any error introduced by the main amplifier.
In another aspect of the invention, there is provided a method for generating and receiving a pilot tone for a feed forward amplifier having a main amplifier, an error correction pathway, and an output pathway, the method comprising the steps of:
producing an oscillating signal;
applying a vector modulation to the oscillating signal to produce a pilot tone;
injecting the pilot tone prior to the main amplifier;
mixing the oscillating signal with a measured output of the output pathway to produced a mixed signal;
filtering an intermediate frequency from the mixed signal to produce a filtered signal; and,
detecting the pilot tone from the filtered signal.
The present invention can reduce expense and complexity over certain prior art FFAs. For example, in various aspects of the present invention, a feed forward amplifier is provided that only requires a single oscillator to generate a pilot tone and provide an oscillating signal for pilot tone receiver circuitry. In contrast, certain prior art FFAs can require two oscillators; one for the pilot tone generator and one for the pilot tone receiver. In another embodiment of the invention, a method is provided of generating and detecting a pilot tone from a single oscillating signal.